I made a small Google Trends analysis of Flight Sim popularity: https://trends.google.com/trends/explore?date=all&q=%2Fm%2F014qwh,%2Fm%2F0dwp83,%2Fm%2F0238h5,%2Fm%2F0j9mhpy,%2Fm%2F0kwh4
Interesting points:
1-Interest in flight simulators is decreasing(compared to 2004)
2-FSX is king(still) but decreasing following the decreasing Flight sim interest in general.
3-X-Plane(total?) gaining more than P3D
4-FlightGear dropping(since 2004)
5-Most interest in FlightGear from Europe
6-YouTube searches for FlightGear are flat(since 2008)